Year	Percent
1960	2.49973
1961	2.71982
1962	2.95738
1963	3.21347
1964	3.49047
1965	3.78795
1966	4.1087
1967	4.45223
1968	4.81903
1969	5.20646
1970	5.61551
1971	6.04668
1972	6.50001
1973	6.96637
1974	7.43941
1975	7.91185
1976	8.38492
1977	8.85951
1978	9.34537
1979	9.84904
1980	10.38065
1981	10.93671
1982	11.52854
1983	12.17474
1984	12.90322
1985	13.72809
1986	14.68032
1987	15.76373
1988	16.95616
1989	18.2051
1990	19.48868
1991	20.77775
1992	22.10826
1993	23.57263
1994	23.9961
1995	23.73942
1996	23.74767
1997	23.96353
1998	24.30082
1999	24.6326
2000	24.85909
2001	24.93739
2002	24.88372
2003	24.71786
2004	24.68509
2005	24.75517
2006	24.79385
2007	24.77664
2008	24.63467
2009	24.28592
2010	23.7146
2011	23.92805
2012	24.84446
2013	25.76424
2014	26.823
2015	28.10696
2016	29.6453
2017	29.71144